1. Field of the Invention
This invention relates to data networks for providing communication between computers, computer peripherals and/or other digital apparatus, and more particularly to ring networks in which discrete frames of data are passed successively to each station on the network, in a predetermined order, until they eventually return to the station that originated them for removal; the stations are thus organised in a logical closed ring (which may, but is not necessarily, reflected in the physical connections). Networks of this kind in current use include the well known "token ring" networks in which access to the network is controlled by a special frame (called a "token") which must be "captured" by a station which has data to transmit and returned to the network when transmission is finished. In some networks (depending on the network protocol) a station holding the token may continue transmitting (or wait) until its own first frame returns to it; in others it may transmit only one frame before returning the token.
2. Description of Related Art
There are obvious security risks in a network in which all the data passes through every station, and it is an object of the invention to permit the construction of networks which retain the essential characteristic of ring networks (so that standard token ring stations, for example, can be used) and yet in which a data frame can be read only by a station to which it is addressed or at least only by stations in the same security group, and/or which is authorised to receive it.